DISCONTINUOUS IGNEOUS ADDITION AND EARLY MARGIN SEGMENTATION ALONG THE EASTERN NORTH AMERICAN MARGIN’S RIFT TO DRIFT TRANSITION
Tomographic modeling from ocean bottom seismometer data on lines 4A and 4B of the ENAM CSE shows a P-wave velocity structure resembling extremely thick oceanic-type crust. We observe minimal along-strike variations in crustal thickness, but substantial variations in the extent of HVLC. Within the HVLC sections, velocities at the base of the crust reach ~7.5 km/s, while two ~30 km-wide gaps are present where velocities only reach ~7.0 km/s. The gaps in HVLC align with isostatic gravity lows and with extrapolated Mid-Atlantic Ridge fracture zones. Mantle melting models indicate that the >7.5 km/s HVLC most likely represents heavy igneous intrusion into extended continental crust while the HVLC gaps most likely represent nearly unaltered extended continental crust. The along-strike intrusion variability also affects the volume of igneous addition, reducing it by up to 76% from previous whole margin estimates. Our along-strike observations closely match fracture zone formation modeling from the less magmatic Woodlark Basin where magmatic centers undergo the rift to drift transition and then connect into a stable ridge configuration. It is unclear what influenced segmentation of the Pangean rift, but it appears that the segmentation of the early rift resulted in a lower volume of igneous addition and directly influenced Mid-Atlantic Ridge segmentation.